I've been told by a Rioter on Reddit that the general policy for VA work is that they remain anonymous unless they themselves request to be officially credited somehow or take public credit themselves- I found this out after asking if Riot could officially credit Lamb's voice actress and commented on how we have multiple champions with great voice work but we have no idea who voiced them and can't send them a letter of recognition for their work, Lamb being the latest at the time (but the voice actress upon Kindred's release publicly announced her identity and thanks for the praise she'd received).
In Evelynn/Syndra's case, the Voice Actress remains anonymous at her disposal. If she wishes to remain anonymous, it's her choice and nobody has to justify her wishes- it's her choice and speculating why someone would wish for that is at best a little disrespectful in my opinion.